I. Field of the Invention
The present invention relates to a method for manufacturing a Schottky barrier diode.
II. Description of the Prior Art
A Schottky barrier diode (SBD) is based on a rectifying barrier (Schottky barrier) formed by contact between a metal and a semiconductor. Since injection of the minority carriers does not substantially occur and a forward bias voltage thereof is smaller than that of a p-n junction diode, the SBD has been widely used as a high-speed clamping diode for the p-n junction diode. A Schottky barrier diode clamped npn transistor is a typical example of such application. TTL circuits with the npn transistor of this type are most widely used as high-speed logic circuits.
In order to manufacture the SBD of this type, a contact hole is formed in, for example, a silicon dioxide insulating film formed on an n-type silicon substrate, exposing a surface portion of the substrate. A metal is deposited on the silicon dioxide layer and the exposed portion of the n-type silicon substrate. Annealing is then performed to alloy the surface silicon of the substrate with the metal. Thus a metal silicide layer is formed. The metal layer portions remaining on the surface of the silicon dioxide layer and on the inner wall of the contact hole are removed, leaving the silicide layer. Finally, a wiring metal layer is deposited on the remaining silicide layer and the silicon dioxide layer portion near around the contact hole.
If aluminum is used as the wiring metal layer, the silicide layer need not be performed, and aluminum may be directly deposited as the wiring metal layer on the exposed surface of the n-type silicon substrate. Thereafter, annealing is performed to alloy part of the aluminum layer with the substrate silicon. Thus, the desired silicide layer can be obtained. However, in this case, the silicon substrate must have the (111) plane on the surface so as not to form alloy pits between aluminum and silicon.
Independently of the Miller index of the silicon substrate, platinum can be used to form a desired silicide layer by alloying with silicon. Since platinum silicide reacts with aluminum used for the wiring metal layer which is formed later on, electrical characteristics of platinum silicide are inadvertently changed. In order to prevent this reaction, a reaction preventing metal such as titanium and tungsten must be deposited on platinum silicide before aluminum is deposited thereon. If such a countermeasure is taken for platinum silicide, a highly reliable SBD can be obtained. Thus, platinum silicide has been recently used frequently in place of aluminum silicide.
However, in the above method for manufacturing the SBD with a platinum silicide layer, silicon in the platinum silicide layer is oxidized by aqua regia used to pattern the platinum silicide, i.e., to remove the platinum, resulting in the formation of a thin silicon dioxide layer on the platinum silicide layer. If hydrofluoric acid is used which is effective for etching silicon dioxide, it etches not only the silicon dioxide on the platinum silicide layer, but also the silicon dioxide defining the inner wall of the contact hole. As a result, the surface part of the silicon substrate which is not alloyed with the platinum is newly exposed. Subsequently, if the reaction preventing metal is deposited, it directly contacts the newly exposed surface portion of the silicon substrate. Thus, in the resulting structure, an SBD formed by platinum silicide and silicon and an SBD formed by the reaction preventing metal and silicon constitute a parallel diode. The barrier height .phi..sub.B of the structure as a whole and hence the forward bias voltage V.sub.F vary depending on the area of the newly exposed portion of the silicon substrate. Since the area can hardly be kept constant, the forward characteristics cannot be obtained with good reproducibility. Further, the SBDs obtained by the above method may vary in a reverse breakdown voltage.
In order to eliminate the above drawback, it has been practised to form a guard ring, which constitutes the p-n junction with the substrate, in the portion of the silicon substrate corresponding to the surrounding region of the contact hole formed in the insulating film. The forward and reverse characteristics are thus improved. However, when the guard ring is formed, the packing density of the SBD is lowered. The SBD of this type is not suitable for an internal element of a highly integrated device. Thus, the SBD is only used in a transistor or diode of an input/output section.
The forward current J of an SBD is expressed as follows: EQU J=Js exp ((qV.sub.F /nKT)-1)
where Js is the saturation current expressed by A*T.sup.2 exp (-q.phi..sub.B /KT), V.sub.F is the forward voltage, K is the Boltzmann's constant, n is the constant representing the junction state, A* is the effective Richardson's constant, q is the electric charge, T is the temperature, and .phi..sub.B is the barrier height. As is apparent from the above equation, in an SBD, if the barrier height .phi..sub.B is great, the saturation current J.sub.s becomes small and the forward voltage V.sub.F is high. However, if the barrier height .phi..sub.B is decreased, the saturation current J.sub.s is increased and the effective forward voltage V.sub.F is dropped. Therefore, when the barrier height .phi..sub.B is controlled with high precision, the forward characteristics of the SBD can be highly precisely controlled.
In order to control the barrier height of the SBD, a method is proposed in J. B. Bindell et al, "IEEE Trans ED", Vol. ED-27, No. 2, P. 420, 1980. According to this method, the electric field strength is extremely increased in the vicinity of the semiconductor substrate surface by ion-implanting an impurity at a high concentration so as to cause a tunneling effect, whereby the barrier height is substantially decreased. The barrier height is thus controlled by the dose of ion-implantation. For example, when phosphor is ion-implanted at a dose of 0 to 1.times.10.sup.13 cm.sup.-2 and at an acceleration voltage 35 KeV, the forward voltage V.sub.F can be changed in a range of 0.4 to 0.1 eV. However, it is described in the above article that the reverse voltage V.sub.R is not substantially dropped as opposed to the change in the forward voltage V.sub.F from 0.4 to 0.25 eV.
However, according to the method just described, if annealing is performed to cure the damage to the semiconductor substrate surface which may be caused by ion-implantation, the doped impurity ions may be diffused to form a new impurity profile. The desired impurity profile determined by the ion-implantation conditions cannot be maintained. Further, since the SBD is basically manufactured by the method described above, the etching of the inner wall of the contact hole in the insulating film remains unsolved.